The present invention relates to an improvement in the method for the preparation of a preform of optical fibers or, more particularly, to an improvement in the method for the preparation of a glassy preform of optical fibers by the deposition of fine silica particles formed by the flame-hydrolysis of a silicon-containing starting material for the glass in an oxyhydrogen flame.
As is known, a preform of optical fibers of, for example, fused silica glass is prepared by the deposition of silica soot or fine particles of silica, formed by the combustion of a silicon-containing starting gas such as silicon tetrachloride which is introduced into a concentric multiplex-tube burner nozzle together with oxygen gas and hydrogen gas to form an oxyhydrogen flame in which the silicon-containing starting gas is flame-hydrolyzed and converted into silica particles, on a substrate body of, for example, synthetic fused silica glass to form a porous body of silica followed by subsequent heating and vitrification of the porous body at a high temperature into a transparent silica glass preform. This process is called the outer-deposition CVD method or VAD method.
The concentric multiplex-tube burner nozzle used here has several nozzle openings including the center nozzle opening and a plural number of ring-wise nozzle openings from which the silicon-containing starting gas for the glass material, inert diluent gas, hydrogen gas and oxygen gas are ejected either separately or as a mixture to cause burning of an oxyhydrogen flame and flame hydrolysis of the starting gas for the glass material in the flame. One of the serious problems encountered in the continuous running of this flame-hydrolysis reaction is that the nozzle end of the burner is sometimes overheated by the oxyhydrogen flame resulting in premature damage or deformation of the burner nozzle if not to mention the problem that the fine silica particles produced from the silicon-containing starting gas by the flame hydrolysis are deposited more or less on the nozzle end of the burner eventually resulting in blocking of the nozzle openings by the growth of the silica particle deposition.
In view of the above mentioned problems, an improved method is disclosed in Japanese Patent Publication No. 61-44822, according to which an inert gas is ejected from a ring-wise nozzle opening of a concentric multiplex-tube burner nozzle which is between two nozzle openings, one, for the ejection of hydrogen gas or a mixture of a silicon-containing gas and hydrogen gas and, the other, for the ejection of oxygen gas. This method, however, has a practical disadvantage that the efficiency of combustion to form an oxyhydrogen flame cannot be high enough because, when the rate of ejection of the silicon-containing starting gas is increased with an object to improve the productivity, the flow rate of the inert gas also must be increased correspondingly since otherwise the nozzle openings are readily blocked by the rapid deposition of silica particles thereon. This problem of silica particle deposition on the nozzle end can be partly solved by providing an intermediate nozzle opening for the ejection of an inert gas between two nozzle openings, one, for the ejection of a mixture of oxygen and the silicon-containing starting gas and, the other, for the ejection of hydrogen gas though at the sacrifice of the efficiency of the reaction to cause a decrease in the effective yield of silica deposition on the substrate.